Prostate cancer (PCa) is the most frequently diagnosed cancer among American males, accounting for 27 % of new cancer diagnoses, but it is directly responsible for only 9.5 % of cancer related deaths. Two recently published clinical trials suggest that overdiagnosis and overtreatment of potentially insignificant cancer is a major drawback of prostate cancer screening, and point to the need for a more specific screening tool that can identify clinically significant prostate cancer. The US-based Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial failed to show a mortality benefit from screening and treatment of prostate cancer. The European Randomized Study of Screening for Prostate Cancer (ERSPC) suggests that with the current standard of care, 1410 men must be screened and 48 additional cases of prostate cancer treated to prevent one death from prostate cancer. Based upon these trials, the U.S. Preventive Services Task Force (USPSTF) has issued a grade D recommendation against PSA-based screening for PCa. Contrast-Enhanced Ultrasound: Recent studies have demonstrated that contrast-enhanced harmonic imaging (HI) can identify enhancement related to vascular flow in higher grade PCa, and that HI can selectively detect patients with clinically significant PCa that are most likely to benefit from therapy. Subharmonic imaging (SHI) is a newer technology for contrast-enhanced ultrasound imaging that provides a 10-fold improvement in the signal to background ratio for visualization of small vascular structures, but has not been previously implemented on a transrectal ultrasound probe. SHI of the prostate is expected to advance the imaging of prostatic neovascularity and to improve selective biopsy of clinically significant PCa. Objective: To adapt SHI to a transrectal probe appropriate for prostate imaging and biopsy (over the first 6 months of this study). The subsequent 18 month pilot clinical study will enroll 50 participants who have a clinical indication for prostate biopsy to quantify SHI for the detection of clinically significant PCa (defined as: Gleason score ? 7, a single core with > 50 % involvement, or > 25 % of biopsy cores positive for PCa). Approach: Each participant will undergo a transrectal ultrasound evaluation of the prostate with conventional grayscale and color Doppler imaging, as well as contrast-enhanced imaging with color Doppler, HI and SHI approaches. Imaging results from each of these techniques will be recorded, but only the SHI findings will be used to guide a targeted biopsy of the prostate. A maximum of 6 targeted biopsy cores will be obtained from each participant, based upon suspicious areas identified with SHI. Following the targeted biopsy, each participant will also receive a 12-part systematic biopsy consisting of 6 laterally directed biopsy cores and 6 medially directed biopsy cores. The pilot study is designed to estimate the detection of clinically significant cancer that will be obtained with targeted biopsy based upon SHI along with the medially directed systematic sextant biopsy cores (? 12 cores total).